Mashing

Just wondering if anything bad comes from boiling water off of your wort from mashing? Would having to boil off excess water hurt any of the fermentables or does the sugar content stay the same, you'll just have a not-so-exact conversion? I havent exactly "mashed" before and Im curious if I could let a gallon or two boil off if that ever happened and the repercussions of that.

I'm considering going grain and building one of those awesome rubbermaid tuns. I saw someone online who did so fairly easily and when they had their wort all mashed, they simply just let the excess water from mixing with the sparge, evaporate by boiling..

I might start off by mashing in my brewpot, keeping the grain in a bag, and using another pot for the sparge.

Will that work ok?

Brew in a bag is a very popular first jump for would-be all grainers to go with. As for boil-off, you generally account for how much will boil off over a given period and sparge enough to hit your mark (5 or 10 gallons generally). Some styles work better with prolonged boils, since the longer it boils the more concentrated the wort becomes. There are also maillard reactions that occur, making melanoidins that increase the malt flavor of the beer.

Oh.. alright. Deer in headlights lol